The present invention relates to an objective lens unit for an endoscope, and a method of assembling the objective lens unit.
Conventionally, a confocal probe, which employs principle of confocal micrography, has become widely known. The confocal probe is configured to illuminate in vivo tissues inside human cavity, and selectively receive light reflected by the tissues on an object side focal plane. The confocal probe is provided with a-specific optical system (i.e., a confocal optical system) which has a relatively high magnification. Recently, a confocal endoscope system which is a combination of the confocal probe and an endoscope has been suggested. An example of such a confocal probe system is disclosed in Japanese Patent Provisional Publication P2000-1121961A.
Generally, an objective lens unit provided at the tip portion of the confocal probe or the endoscope is configured to have a plurality of lenses constituting an objective optical system and a lens barrel holding the plurality of lenses. An exemplary structure of such an objective lens unit is disclosed in Japanese Patent Provisional Publication P2000-75218A.
When such a small objective lens unit for precision instruments such as the confocal probe and the endoscope is assembled, a worker pinches lenses one by one using tweezers in accordance with a procedure, and drops the lens elements one by one in the lens barrel and locate them in predetermined positions of the lens barrel. Then, the worker places holding rings to fix the lenses inside the lens barrel.
The confocal probe is provided with an optical system similar to that of the confocal microscope, which is configured to observe an object at high magnification with high resolution. For this characteristic, the objective lens unit of the confocal probe is provided with a plurality of lenses, and possible aberrations are minimized.
The confocal probe has a small outer diameter, and many lenses which also have small diameters. Therefore, the lens barrel for the optical system of the confocal probe has a thin and elongated cylindrical shape. When the objective lens unit such as one for the confocal probe accommodating many lenses, as the worker inlets the lenses one by one, the lenses which are fitted in the lens barrel at an early stage of the assembling operation (i.e., the lenses located at a bottom side of the lens barrel) are located below the lenses assembled in later stage. Accordingly, between the lenses at the bottom side of the lens barrel and the worker, there area a plurality of lenses. Thus, the amount of light incident from the upper part of the lens barrel is lowered at the bottom side of the lens barrel by the plurality of lenses. Further, the lens barrel to which the lenses are fitted is formed to have an elongated shape. Therefore, light of the microscope entering from the above hardly reaches the bottom side of the lens barrel. Thus, during the assembling process, it is difficult for the worker to check the status of the lenses at the bottom side of the lens barrel. As a result, even if the lenses at the bottom side are misaligned, the worker may not recognize the misalignment. Such an assembling error sometimes causes deterioration of optical performances of the optical system.